The Notch signaling pathway regulates a diverse array of cell functions (Kopan et al., Cell 137, 216-233 (2009)). Four Notch receptors have been identified in mammals, i.e., Notch 1-4, that share basic structural elements that include an extracellular domain, a transmembrane domain, and an intracellular domain. Similarly, the canonical ligands of Notch share certain structural similarities but a number of non-canonical ligands of Notch have also been identified (Kopan et al., Cell 137, 216-233 (2009)). The five canonical ligands in mammals are Delta-like 1, Delta-like 3, Delta-like 4, Jagged1 and Jagged2. Binding of a Notch ligand to the extracellular domain of a Notch receptor sets a signaling cascade in motion that begins with proteolytic cleavage at the extracellular S2 site by an alpha secretase of the ADAM (a disintegrin and metalloprotease) family. Cleavage at S2 is followed by proteolytic cleavage by a gamma secretase at the intracellular S3 site, which results in release of the intracellular domain and downstream events that ultimately activate Notch-dependent transcription factors such as Hes1 and Hey.
Because aberrant Notch expression and signaling has been implicated in a number of diseases, including cancer (Koch et al., Cell. Mol. Life Sci. 64, 2746-2762 (2007)), modulators of Notch signaling have been investigated as possible therapeutic agents for such diseases. For example, gamma secretase inhibitors have been tested in clinical trials for their effectiveness in treating various malignancies (Shih et al, Cancer Res. 67, 1879-1882 (2007)). Gamma secretase inhibitors prevent cleavage at S3 and thereby prevent signaling through Notch receptors. However, gamma secretase inhibitors do not distinguish individual Notch family members and therefore inhibit signaling through multiple receptors at once, as well as through unrelated pathways (Beel et al., Cell. Mol. Life Sci. 65, 1311-1334 (2008)). Consequently, administration of gamma secretase inhibitors is associated with intestinal toxicity marked by weight loss and intestinal goblet cell metaplasia, indicative of a role for Notch in determining cell fate by maintaining proliferation of intestinal crypt progenitor cells and prohibiting differentiation to a secretory cell fate (See van Es et al., Nature 435:959-963 (2005)). Similarly, inhibition of both Notch1 and Notch2 signaling via conditional Notch gene knockout (Riccio et al., EMBO Rep. 9:377-383 (2008)) or via antagonist antibody inhibition (US Patent Application Publication No. 2010/0080808) also causes intestinal goblet cell metaplasia.
Because of serious toxicity associated with inhibitors of multiple Notch receptors, there is a great need in the art for targeted inhibition of signaling through specific receptors.